Do not wrap AsyncIterator and Coroutine with extra Coroutine in async functions

[sterliakov]

Experiment for https://discuss.python.org/t/overloads-of-async-generators-inconsistent-coroutine-wrapping/

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
- steam/leaderboard.py:114: error: Overloaded function implementation cannot produce return type of signature 1  [misc]
- steam/leaderboard.py:114: error: Overloaded function implementation cannot produce return type of signature 2  [misc]

jinja (https://github.com/pallets/jinja)
+ src/jinja2/filters.py:1520: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ src/jinja2/filters.py:1571: error: "return" with value in async generator is not allowed  [misc]
+ src/jinja2/filters.py:1607: error: "return" with value in async generator is not allowed  [misc]
+ src/jinja2/filters.py:1647: error: "return" with value in async generator is not allowed  [misc]
+ src/jinja2/filters.py:1685: error: "return" with value in async generator is not allowed  [misc]

websockets (https://github.com/aaugustin/websockets)
+ src/websockets/legacy/protocol.py:1264: error: Incompatible types in "await" (actual type "float", expected type "Awaitable[Any]")  [misc]

starlette (https://github.com/encode/starlette)
+ tests/middleware/test_gzip.py:70: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ tests/middleware/test_gzip.py:75: error: Argument 1 to "StreamingResponse" has incompatible type "Coroutine[Any, Any, Union[AsyncIterable[Union[str, bytes, memoryview]], Iterable[Union[str, bytes, memoryview]]]]"; expected "Union[AsyncIterable[Union[str, bytes, memoryview]], Iterable[Union[str, bytes, memoryview]]]"  [arg-type]
+ tests/middleware/test_gzip.py:75: note: Maybe you forgot to use "await"?
+ tests/middleware/test_gzip.py:94: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ tests/middleware/test_gzip.py:100: error: Argument 1 to "StreamingResponse" has incompatible type "Coroutine[Any, Any, Union[AsyncIterable[Union[str, bytes, memoryview]], Iterable[Union[str, bytes, memoryview]]]]"; expected "Union[AsyncIterable[Union[str, bytes, memoryview]], Iterable[Union[str, bytes, memoryview]]]"  [arg-type]
+ tests/middleware/test_gzip.py:100: note: Maybe you forgot to use "await"?

trio (https://github.com/python-trio/trio)
+ src/trio/_tests/test_util.py:150: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]

scrapy (https://github.com/scrapy/scrapy)
+ scrapy/utils/defer.py:344: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ scrapy/spiders/crawl.py:166: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ scrapy/core/spidermw.py:129: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ scrapy/core/spidermw.py:146: error: Incompatible return value type (got "Coroutine[Any, Any, AsyncIterable[_T]]", expected "Iterable[_T] | AsyncIterable[_T]")  [return-value]
+ scrapy/core/spidermw.py:146: note: Maybe you forgot to use "await"?
+ scrapy/core/scraper.py:284: error: Incompatible types in assignment (expression has type "Coroutine[Any, Any, AsyncIterable[_T]]", variable has type "Iterable[_T] | AsyncIterable[_T]")  [assignment]
+ scrapy/core/scraper.py:284: note: Maybe you forgot to use "await"?
+ scrapy/core/scraper.py:288: error: Argument 1 to "parallel_async" has incompatible type "Iterable[_T] | AsyncIterable[_T]"; expected "AsyncIterable[Any]"  [arg-type]
+ scrapy/spidermiddlewares/urllength.py:42: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ scrapy/spidermiddlewares/referer.py:393: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ scrapy/spidermiddlewares/offsite.py:52: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ scrapy/spidermiddlewares/depth.py:54: error: The return type of an async generator function should be "AsyncGenerator" or one of its supertypes  [misc]
+ scrapy/commands/parse.py:159: error: Argument 1 to "collect_asyncgen" has incompatible type "Coroutine[Any, Any, AsyncIterable[Any]]"; expected "AsyncIterable[Never]"  [arg-type]
+ scrapy/commands/parse.py:159: note: Maybe you forgot to use "await"?

discord.py (https://github.com/Rapptz/discord.py)
+ discord/state.py:601: error: Argument 1 to "wait_for" has incompatible type "list[Member]"; expected "Future[Never] | Generator[Any, None, Never] | Awaitable[Never]"  [arg-type]
+ discord/state.py:1234: error: Overloaded function implementation cannot produce return type of signature 2  [misc]
+ discord/state.py:1797: error: Argument 1 to "wait_for" has incompatible type "list[Member]"; expected "Future[Never] | Generator[Any, None, Never] | Awaitable[Never]"  [arg-type]